How To Store Data On A Computer (For A Computer)

Transcription

1 TH3. Data storage A computer uses two types of storage. A main store consisting of ROM and RAM, and backing stores which can be internal, eg hard disk, or external, eg a CD or USB flash drive. ROM and RAM Main store (or computer memory) is divided into Read Only Memory (ROM) and Random Access Memory (RAM). ROM ROM is memory that cannot be changed by a program or user. ROM retains its memory even after the computer is turned off. For example, ROM stores the instructions for the computer to start up when it is turned on again. RAM RAM is a fast temporary type of memory in which programs, applications and data are stored. Here are some examples of what's stored in RAM: the operating system applications the graphical user interface (GUI) If a computer loses power, all data stored in its RAM is lost. Storage capacity and file size Storage capacities and file sizes are measured from lowest to highest in: 1

2 bits bytes kilobytes megabytes gigabytes terabytes An operating system abbreviates these measurements, eg 1 megabyte becomes 1MB (megabyte). Relationship between measurements The table below outlines the relationship between bits (smallest) and terabytes (largest). Read each row of the table from left to right: Relationship between measurement Size Equal to 8 bits 1 byte 1024 bytes 1 kilobyte 1024 kilobytes 1 megabyte 1024 megabytes 1 gigabyte 1024 gigabytes 1 terabyte The size of a file and a storage device's capacity will always be written in its simplest form. For example, an operating system would report a 1 terabyte hard disk's size as 1TB not as 1024GB, although both are correct. Example file sizes The table below lists files commonly found on a computer and their typical file size (compressed): File File size Photo 3MB Song 5MB Film 700MB A file s size can be influenced by a number of factors but ultimately the more information a file stores, the larger it will be. The hard disk The main internal backing store is a computer's hard disk. Hard disk 2

3 The top side of a hard disk A hard disk stores: the operating system software applications or programs the majority of your data files Hard disks spin at very high speeds (around 7,200 RPM - revolutions per minute) within a sealed unit inside the computer. Hard disks store large amounts of data - 200GB to 1TB is common in desktop computers. The data stored on a hard disk is retained until deleted, but it needs to be loaded into main store RAM before it can be used. External backing stores: optical discs There are several different types of optical disc, although they all look pretty much the same. CD (Compact Disc) Optical discs that use the same technology as music CDs. They store up to 700MB of data. CDs can be used for multimedia applications such as encyclopaedias and can store pictures, sounds and video clips or anything else that will fit. There are several formats on the market, such as: CD-ROM - read only, the data is written to them before they are sold. CD-R - meaning CD-Recordable, the user can write data to the CD once or fill it over time using multi-session (writing to the same disc on separate occasions). CD-RW - meaning CD-ReWritable, the CD can be written and re-written to. Unlike multi-session discs, existing data can be overwritten. DVD (Digital Versatile Disc) DVDs are the same physical size as CDs but hold much more data - a single sided disc can hold up to 4.7GB. DVDs are commonly used for storing video so you will often see them measured in minutes, eg 4.7GB = 120 minutes. 3

4 There are several formats on the market, such as: DVD-ROMs - read only, the data is written to them before they are sold. DVD-R - meaning DVD-Recordable, the user can write data to the DVD once or fill it over time using multi-session. DVD-RW - meaning DVD-ReWritable, the DVD can be written and re-written to. Unlike multi-session discs, existing data can be overwritten. CD/DVD drives To read from and write data to CDs and DVDs you will need a suitable drive. Today you can buy CD/DVD drives that are able to: read all CD and DVD formats write to CD-R, CD-RW, DVD-R and DVD-RW Data is written to and read from the discs using a laser. External backing stores Magnetic tape Principally used for backup of important files from the hard disk and for the long term storage and archiving of data. Advantage They are small, robust, portable and provide low cost storage per GB. Disadvantages They are very slow to write to and read from. Serial access means all the data on the tape must be read before you can access the data you need. External hard disks An external hard disk can store very large amounts of data, eg 1TB, and can be plugged into your computer via a USB or FireWire port to provide extra storage. They're essentially the same as the hard disk in your computer but due to the casing are usable externally. 4

5 Advantage High capacity, eg 1TB or more so they can be used to backup data or move large amounts of data between machines. Disadvantages They're often quite large and therefore inconvenient to carry around. They have moving parts so are more likely to break, especially if dropped. Memory sticks A memory stick is 'pen top' sized USB device and can be used in a similar way to a floppy disk but it is inserted into the USB port - it is then seen by the computer as a removable drive. They typically come in sizes from 512MB to 32GB upwards depending on the price paid. Advantages Memory sticks can hold large quantities of data. They are extremely portable, so the user can take them wherever they go. They're durable, because they have no moving parts. Disadvantage Portable storage devices in general are more likely to be lost, stolen or damaged. Memory Cards A memory card is a stamp-sized USB device and can be used in a similar way to a floppy disk but it is inserted into either a memory card reader or a USB converter - it is then seen by the computer as a removable drive. They typically come in sizes from 1 GB to 32 GB upwards, depending on the price paid. There are many available memory card formats, but since 2010 the SD card became the more favoured format. Advantages Memory cards can hold large quantities of data. They are extremely portable so the user can take them wherever they go. Digital accessories such as compact cameras and mobile phones are able to read and write to memory cards allowing the user to transport large collections of photographs, songs or information with them. They're durable because they have no moving parts. 5

6 Disadvantage Memory cards, specifically the micro SD card, are the smallest storage devices available. This means they are more likely to be lost, stolen or damaged. All computers do not come with memory card readers built in. Users will often be required to purchase a card reader or USB converter to view the data on a card. Backing up and archiving data Data needs to be backed up for many reasons: a user may delete an important file hard disks can fail a virus can wipe out data a fire may destroy the building where the data was being stored (businesses will often store their backups off-site) If the data wasn t backed up then the consequences could be disastrous depending on what data was lost. If a business lost details of all the payments it had yet to receive the business could go bankrupt as they wouldn t know what was owed to them or by who. Businesses typically use magnetic tape to backup important data. Your average home user is unlikely to backup their data but those who do will most likely use an external hard disk. Frequency of backups Some data is more valuable than other data and some data is changed more frequently than other data. These are the kind of issues that must be taken into account when deciding how often to backup data. The value of the data should determine how frequently it is backed up. If the data doesn t change often then it doesn t need to be backed up as often, maybe just after each change. If the data changes frequently then it should be backed up frequently (maybe every evening). Links The BBC Webwise guide to backing up your data. Archiving Some data may not be being used very often but it may still be useful or needed in the future. In this case data can be archived. Archived data is copied to a suitable storage medium (perhaps DVDs or magnetic tape) then it is stored safely and securely. The original data is then deleted from the computer system. This is done to free up storage space for new data. 6